UV mapping and removing hidden faces?
If you're looking for a quick way to delete faces that will never ever be visible from the outside, you're in luck. If you're trying to do something more complex, you're going to be stuck with manual work.

To delete these inside faces, select everything using A. Then switch into face mode Ctrl-Tab>Face. Set your camera to orthographic Numpad-5. Finally enter each one of the side/top views, use Box select with MMB to de-select all the visible faces. You will need to do this for all 6 views: The short cuts are 1,3 and 7 for front, right and top (respectively) To get back left and bottom, combine  1,3 and 7 with Ctrl to get back, left and bottom (respectively). The selected faces you will now have are the inside faces. Delete them.